Thin films are formed for example for the purpose of producing GMR (Giant Magnetoresistance) or TMR (Tunneling Magnetoresistance) elements in memories. GMR elements have at least two ferromagnetic layers between which is arranged a nonmagnetic, conductive layer that exhibits the so-called GMR effect. In the case of the latter, the electrical resistance due to the three layers or thin-film layers is dependent on the orientation of the magnetization in one of the two ferromagnetic layers relative to the magnetization direction of the other ferromagnetic layer. In this case, a parallel or antiparallel orientation may result as stable states of the magnetization.
TMR elements have at least two ferromagnetic layers between which an insulating, nonmagnetic layer is arranged in each case. The latter is patterned thin enough such that a tunneling current from a first to a second of the ferromagnetic layers may occur. In a similar manner to the GMR elements, the magnetoresistive effect consists of having different resistances depending on a parallel or antiparallel orientation of the magnetization of the two ferromagnetic layers. The spin-polarized tunneling current flowing through the thin layer arranged between the two ferromagnetic layers is subject to larger resistance changes, which may amount to up to 40%, in the case of the TMR effect compared with the GMR effect.
The above-mentioned GMR and TMR elements may advantageously be used in memory cells for storage of a binary information item. While one of the two ferromagnetic layers is usually patterned with an essentially invariable magnetization direction, the second ferromagnetic layer has the property of being able to be reprogrammed from the parallel to the antiparallel state or vice versa by means of an externally applied magnetic field of sufficient strength. Such a magnetic field may be formed for example by means of the magnetic fields that are generated and superposed by word and bit lines that cross one another. Accordingly, a memory cell is programmed by means of corresponding driving of the word and bit lines.
German patent application DE 199 08 519.6 or M. N. Yoder, “Microelectronics-Nanoelectronics and the 21st Century”, p. 2-7, IEEE 2001, discloses annular monolayer or multilayer thin-film systems as GMR or TMR elements. They afford the advantage of a closed magnetic flux within the rings, so that magnetic interference fields occur toward the outside beyond the local region of the memory element only during a magnetization reversal process that is carried out for the purpose of programming. Since it is thus possible, in particular, also for adjacent magnetoresistive memory elements to be magnetically decoupled, it is theoretically possible to achieve higher integration densities of thin-film layer microstructure elements in memory cell arrangements.
The possibility of achieving higher integration densities in the prior art is limited, however, by the below-described disadvantages of the conventional methods for producing microstructure elements based on thin films.
Firstly, the monolayer or multilayer thin-film system is deposited over the whole area onto a substrate comprising, by way of example, monocrystalline silicon and a number of patterned layers arranged on the silicon. The thin-film system comprises at least three layers in the case of GMR or TMR elements. Afterward, the thin-film system is patterned by means of lithographic and etching processes in such a way that all that remains are the annular microstructures comprising, by way of example, the at least three layers of the memory elements. In the lithography step, a photosensitive resist with the desired annular form, which is arranged on the thin-film system, is exposed and developed. In the etching step, the structures produced in the resist are transferred into the thin-film system. For this purpose, it is possible to use known plasma etching methods such as, for example, RIE (Reactive Ion Etching), ECT (Electron Cyclotron Resonance), ICP (Inductively Coupled Plasma), CAIBE (Chemically Assisted Ion Beam Etching) methods, etc.
However, the materials that comprise the ferromagnetic layers such as iron, nickel or cobalt, lead as reaction products, on account of their lack of volatility, to a disadvantageous redeposition of metallic layers at the etching sidewalls of the microstructures formed. This may in turn lead to short circuits arising at the outer edges of the microstructures. A further disadvantage arises from the fact that, on account of the intensified ion bombardment required for etching the ferromagnetic layers, there is only low selectivity during the etching of the thin film in comparison with the etching mask, for example of the resist or a hard mask, and also with respect to a further metallic or dielectric support. The latter may comprise copper, tantalum or silicon dioxide, for example. Therefore, particularly thick resist or hard masks are required, which in turn leads to a loss of dimensional accuracy during the structure transfer and to oblique etching sidewalls at the microstructures. Lateral insulation of the TMR or GMR elements by means of dielectric spacers, for example, is thereby made more difficult.
If a chlorine-containing gas is used for the plasma etching process, then a disadvantageous corrosion of the layer system may additionally occur particularly in the case of antiferromagnetic iron-manganese or iridium-manganese.
In order to avoid the described problem with the reaction products, it is also possible to use a so-called damascene technique, in which the structure formation is firstly performed in a dielectric layer that is already present, which, as trench structures, are subsequently filled with the desired films in a deposition process. Layer portions projecting outside the trench or hole structures formed are polished back as far as the surface of the dielectric layer in a chemical mechanical polishing process (CMP), for example. It is also possible to use a homogeneous etching process in this case. A deposition may be carried out for example by means of a physical or chemical vapor deposition process (PVD, CVD) or an electrodeposition.
However, since the layer sequence is deposited vertically at the trench walls particularly in the case of multilayer systems, the polishing process uncovers the outer edge of the thin-film system at the surface of the substrate. This may result for example in short circuits when making contact with interconnects that are subsequently formed.
The German patent application DE 100 50 076.5 describes how microstructures of the thin-film systems can be obtained by sputtering or vapor deposition of the relevant materials in perforated masks which, by virtue of overhangs in the hole profiles, the so-called shadowmasks, are constricted in an upper region in such a way that the sputtered particles can only reach a central region of the base area of the hole structures. The advantage is that the deposited layers do not reach as far as the trench wall since the corresponding particles from the sputtering process cannot reach the trench wall on account of the overhang in the hole profile. Consequently, vertical layer portions that could be uncovered by a CMP process do not arise.
However, it has not yet been possible heretofore to find a suitable process with which the advantageous annular microstructures could be formed by application of this technique. Shaping a hole in the center of the thin-film system for the purpose of forming the ring would necessitate, in the case of high integration densities, shaping a mushroom-shaped structure on account of the overhang, which structure would have a high degree of physical instability, e.g., with respect to the effects of subsequent processes. Therefore, high integration density cannot be achieved by means of this method.